No matter what for us Greeks all were savages and primitives
When Greeks first time came to existence, Slavs (ie Thracians/Sarmatian/Celts) existed on Inter-continental levels for numerous millennia already. Slavs were self born civilization raised and evolving with time from hunters, gatherers, farmers. Slavs also lived as nomads and sedentary people. Most importantly, ancient Slavs had its rural and urban population centers.
Now, compare Greeks to it. It can`t be compared. Greeks came to existence on fringes of Slavic world, as result of cultural and genetic influences from non-Slavs on Slavs. Namely, from Semitic influences on Slavs. Those intermixing happened at first in Slavic villages (preferable on sea shores, or even on some island where additional isolation accelerated extraction of new culture) that evolved into trade centers and where foreign element was concentrated. So, when Greeks came to existence they founded themselves being urban population encircled with Slavic rural population. Until other Slavic urban centers in that particular wide region recognized newly emerging danger, it was obviously too later. Greeks were started to spread and be sure in a first and second steps solely by genocide on neighboring rural Slavic population. Exactly same as how Rome came to existence. Exactly same. Next step was to ruin neighboring competing Slavic urban centers. In case of Rome, it was Etruscan (that called themselves Rasena/Rasna=urban Sarmatians/Thracians/Celts) urban centers.
See, that is why for Greeks and Romans (themselves mix of Slavs and some others), Slavs were backward or to say- other traditional Slavs were backward. They first dealt with Slavic rural population and then systematically executed destruction of Slavic urban centers, depriving Slavs of urban elite and slowly ruining Slavic civilization. How this was possible? It was gradual with time and simple, younger cultures have that momentum of unusual hostility, greed and necessity. At time when these young civilizations came to existence Slavs existed, as linguists confirmed for 10-40.000 years already. Too long they lived in established order of things and were surprised by this young hostile invaders. Invaders, born from with Slavic world itself.
This story repeated not only with Greeks and Romans but also with Germanics on Baltics and some others, too.